Light intensity regulation



Ug- 2, 1949. G. J. PERLOW ET AL LIGHT INTENSITY REGULATION .LE-... L

gmc/whom 'GILBERT J. PERLow GLENN A. JOHNSON Patented Aug. 2, 1949 LIGHT INTENSETY REGULATION Gilbert J. Perlow and Glenn A. Johnson, Washington, D. C.

Application May 3, 1945, Serial No. 591,764

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) 4 Claims.

This invention relates to control of the intensity of illumination from a light source, and in particular it relates to a system, using a photosensitive element and a light source, in which the intensity of light striking the photo-sensitive element can be adjusted to a desired value and be thereafter held essentially constant at that value, notwithstanding substantial changes in external conditions, voltage of power sources, etc.

This invention finds useful application in any situation where control of illumination at a constant Value is desirable, including applications in television, photography, photoelectric sweep circuits for cathode ray tubes and many other elds.

The invention will be further described with reference to the exemplary embodiments shown in the drawings, in which:

Figure l is a schematic drawing of an embodiment in which the controlled light source is a lamp powered by direct current, and

Figure 2 is a schematic drawing showing an embodiment of the invention using radio frequency current to actuate the controlled light source.

Constant intensity electrical light sources must, for many applications, be supplied with direct or high-frequency alternating current, because the use of 60 cycle current results in a flicker or rapid variation in intensity which, while usually invisible to the eye, will seriously interfere with the operation of light-sensitive photographic or electrical apparatus.

Referring to Figure 1, this embodiment of the invention comprises a light source, lamp 4, and a photo-sensitive device, photoelectric cell I. The photoelectric cell is positioned within the zone of stabilized illumination to receive incident light from lamp 4. An illumination control means for varying the light incident on cell I from the lamp is provided. The control means is responsive t0 the cell output, and operates to stabilize the incident flux thereon, varying the light received from the lamp.

In the circuit of Figure l, lamp 4 contains a filament 5, one side of which is connected directly to ground, which is also the negative terminal of source I8. The other terminal of filament 5 is connected to the positive terminal of source IB through resistor IB. In this circuit the incident light is controlled directly by variation in the source intensity. Control tube I I has its cathode i4 connected to ground, which is the negative side of iilament 5 of lamp 4. Plate I2 of tube II is connected to the positive side of filament 5.

Variable resistor I'I is also connected directly across the terminaals of lamp lament 5. Resistor I6, then, carries not only the current drawn by lamp 4 but also the plate current of tube I I and the bleeder current drawn by resistor i'I. Photoelectric cell I is illuminated by light, including light from lamp 4. The anode 3 of cell I may be connected to any convenient positive D. C. voltage source. Here it is shown connected to the midpoint of the voltage divider consisting of resistors 6 and I9, which are connected across D. C. source I8. Cathode 2 of cell I is connected through load resistor 1 to the tap 8 on potentiometer 9, which is a point negative with respect to ground. When light strikes cathode 2 of cell I, cell I passes a current which flows through load resistor 'I and causes a voltage drop therein. Grid I3 of tube II is connected to the less negative end of resistor l.

The system operates as follows: The brilliance of lamp 4 may be controlled by variable resistor Il and potentiometer 9. Variation of resistor II alters the bleeder current and thus affects the total voltage drop across resistor I6. Variation of the movable tap 8 on potentiometer Il changes the grid bias on tube I I and thus affects its plate current, which also influences the total voltage drop across resistor I6. Resistor I'I and potentiometer 9 should be so adjusted that lamp 4 is at appropriate brilliance to give the desired total illumination of cell I and the grid bias on tube II is of such value that the tube is operating near the middle of the linear portion of its grid voltage-plate current characteristic. Any increase in the illumination of cell I causes the current through it to increase, thereby increasing the voltage drop across resistor l and making grid I3 less negative. This increases the plate current of tube II, which increases the voltage drop across resistor I6 and reduces the voltage across lament 5, thus causing lamp 4 to grow less brilliant. Decrease in the illumination of cell I operates conversely to make lamp 4 more brilliant. Consequently the total illumination of cell I, and that of the illuminated zone, is held substantially constant at the desired level.

Figure 2 shows an embodiment of the invention which employs radio frequency current to activate a controlled light source. This embodiment of the invention holds the illumination level constant within very narrow tolerances and offers great exibility with respect to the current and resistance characteristics of the controlled electric light source. In this embodiment tube 39, a screen-grid type tube, is connected as an oscillator, any convenient circuit and any convenient radio frequency being employed. In Figure 2, tube 39 is shown connected as a Hartley oscillator, coil 48 and condenser 46 forming the tank circuit, condenser 45 and resistance 44 serving as grid condenser and grid leak respectively, `and condenser 41 acting as by-pass condenser. In a specic construction employing the invention, an operating frequency of 600 kilocycles was used. The lilament of lamp 24, the controlled light source, is connected by transmission line 26 to coil 49, which is inductively coupled to coil 48 of the oscillator tank circuit. Plate voltage for tube 39 is obtained from source 5|. ,Screen grid 42 vof tube 39 is connected to the plate v34 of tube 33, and both are connected to the positive terminal of source 50 through resistor 31. Condenser 38 is a radio frequency by-pass condenser. Cathode til of tube 36, cathode 390i tube 33, the negative terminals of sources 50,-1:5|, and 21 and the positive terminal of source32 are all connected'to ground. Photoelectricce'll 2| is illuminated'by light, including `light `from lampr24, the controlled light source. The anode 23 of cell '2l is connected to the positive terminal of source 21. Cathode 22 of cell 2| is connected through load resistor 29 to tap 30 on potentiometer 3|, a -point which is negative withrespect to ground. :When light strikes cathode22 of .cell 2l, cell 2| passes acurrent which flows through load resistor '29 and causes a voltage drop therein. Grid of tube 33 is connected-'to the less negative end of resistor l29. Condenser -28 isl a radio frequency by-pass condenser.

.The system operates as follows: The brilliance of lamp 24 may be controlled by moving tap t3!) on 4potentiometer `3|. Movement of -tap 3,0 vto.- ward` the. negative end-of potentiometer.r 3| makes the grid 35 of tube vB3-more negati-valthus reducing the plate current of tube'33. `The reduced current through resistor'3'1 reduces the voltage drop vacross resistor-31 Aand raises the voltage-ap.- pliedtoscreen grid 4:2 offtube'f39. The increased screen voltage raisesfthe output of the ,oscillator of which tube 39 Yis a component, increases the induced `currelitill .coil .149 and thus causes lamp 24 tobecomefmore brilliant.

VIf tap 30 on potentiometer v:3| be moved to.- ward the ground end yofpotentiometer 3|, .converselv the oseille-toroutput-is-reduoed andthe lomp L24 fis made less brilliant. The total illu- Initiation of photoelectrio v:oeil :.2 l :thus ,set :to the-.desired level. Thereafter .an-v; increase in the illumination of sell?! es esthe-curfrentthreuell Cell-2l to increase, .which increases :the voltage across resistor 29-aod1melseserid 3.5 of tube 33 less negative. This intresses the plstecurrent of tube 33, which raisesthe voltage aorossxresistor 31 and lowers the screen voltage ofY tube 3.9. As a result the-osoilletoroutput drops ,and causes lamp 24 to be less brilliant. A decreaseyn illu- Initiation of Cell ,2l operates oonverselvto cause lamp 24 to brighten. Accordingly the total-illu- Initiation of cell ,2l is held verviiearivconstant notwithstanding relatively large Changes Ain eX- ternal conditions o r in the voltage 0i-` thesources employed.

It will be understood thatthe embodiments shown and described .are exemplary rolilv, and

that the scope of the invention will bedetermined with reference to the appended claims.

The invention herein described may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

,1. An illumination control system comprising an electrical light source, a photo-sensitive element positioned to receive light from said light source, oscillator means energizing the light source, and control means responsive to said 4photo-sensitive element operative to control said oscillator vmeans to vary the output intensity of said light source inversely with the intensity of light incident on said photo-sensitive element.

2. An illumination control system comprising an electrical light source, a photo-sensitive element positioned to receive light from said light source, a 4vacuum tube oscillator operative to energize the light source, said vacuum tube includ.- ing ,a control element, and control means responsive to .said photo-sensitive element operative-to control the potential von said control elementof the -said vauum tube to vary the output intensity of said light ysource inversely with the intensity of light incident on said photo-sensitive element,

3. An illumination control system comprising an electrical light source, a photo-sensitive element positioned to receive light from said lig-ht source, a vacuum tube oscillator operative to ,er1- ergig-ze the light source, and `circuit means respon s iveyto said photo-sensitive element operativeto control the amplitude of oscillations of said osejlF later to vary the ouput intensity of said light source inversely with the intensity of lightilli.- dent on said photo-sensitiveelement.

4.An illumination control system comprising anelectrical light source, a photo-sensitive element positioned to receive light from said light source, Va. vacuum tube containing a cathode, .an anode, a ,control grid, `and a screen grid, circuit meansoollpline the vacuum tube electrodes in an oscillator circuit operative to energize the V light source, and circuit means responsive to said photoelectric ,element operative to control the screen grid potential of said vacuum tube to vary the output intensity of said light source inversely with .the intensity of light incident on said photo- Sensitiveelenlent.

G ILBERT J. PERLOW.

GLENN A. JOHNSON.

.REFERENCES CITED The following ref erenlces are of record in the ile of this patent:

UNITED STATES PATENTS Number Name Date 1,107,438 Moskowitz Aug. 18, 1914 1,958,986 Culver May 15,'1934 2,012,821 King Aug. 27, 1935 2,043,671 McMaster June 9, 19,36 2,158,193 Morse May 16, 193,9 2,242,638 Balsley May 20, 1941 2,269,324 Turner et al. Jan, 6, 1942 2,319,287 Arendt May 18, 1943 2,356,195 Balsley Aug. 22,-1944 

